Solar modules are very special products. Compared to other high-tech products, their design is not excessively complicated, and it is not for nothing that they are commodity products, which are produced in large quantities under high price pressure. Nevertheless, it is extremely difficult for investors to separate the wheat from the chaff, to distinguish reliable modules with high performance from those that degrade faster or which have yields that disappoint from the beginning.
Often, no general judgment can be made. Buyers are naturally keen to purchase a module that does not deteriorate even under high tropical humidity, that loses only a little efficiency under weak light conditions, and that still delivers at equatorial temperatures. Technically it is not impossible to achieve all of this with the same module, but investors must also pay attention to the price. Therefore, depending on the utilization of the module, it is important to place more emphasis on certain test categories than on others.
This is one part of the concept driving pv magazine’s new module testing program. Another is the procedure with which the modules are procured from the manufacturers prior to testing. This is extremely important, and this procurement must be well implemented, in order for the results of such a test to be more credible than those from general certification.
Many characteristics are tested during certification, and one could assess the quality of a module based on these. However, the key difference is the procurement. Often, manufacturers send out ‘golden modules’ which perform particularly well in the certification, so the results may not be transferable to the bulk of modules produced in the factory.
This mistrust is justified: “Our first test results prove this: some of the test samples would have never been sent to us, if the manufacturers could hand-pick them”, explains George Touloupas, Director of Technology and Quality at CEA. He developed the test protocols together with pv magazine, and CEA has monitored the complete testing process to ensure highly accurate results. Starting on page 88, he describes the testing methodology in detail, and also explains the ways module buyers can read and interpret the results.
Credibility is one of the essential prerequisites for a test to be accepted, and was the top priority in designing this test. The measurements are carried out in GSolar’s test laboratory in Xi’an, China. To ensure the quality of Gsolar’s tests, pv magazine chose CEA as a partner. CEA acts as an independent third-party expert, and monitors all steps of the testing program to ensure that measurement protocols are respected. pv magazine selected CEA as its partner for this program based on CEA’s considerable expertise in monitoring the quality of large module deliveries for large projects on behalf of investors.
First results
The test pipeline already includes more than a dozen module types from different manufacturers. Three sets of results are partly released here (see p.86). Two manufacturers took the option of a second round of testing, so their results are not published here in detail. pv magazine and CEA decided that it should be possible for these manufacturers to improve.
pv magazine does not only want to support investors and consumers in their module purchasing process, but aims to improve quality in the industry as a whole. Therefore, all test participants are given the choice to either have their full test results published immediately, or opt for a second test round within six months of the completion of the first. In the end, the results are always published completely, thereby creating an increasingly useful database of test results and allowing readers to compare all tested modules. Once the results are completely published, a manufacturer can use the test label with the rating result on its products.
Jolywood is the first manufacturer who opted for the publication of the full results in the first round. The company achieves an “A” rating with its JW-D60N module, on the A + / A / B / C / D scale. As a special feature, the module combines several current technological trends simultaneously: it is an n-type module, it uses PERT technology and is bifacial (see p.87). The results obtained here refer only to installations with system voltages of 1,000 V, as PID testing was carried out at this voltage. Further testing will be carried out to address 1,500 V installations. The second manufacturer, Perlight, also achieved an “A” rating, but the company opted for another test round. The same with Risen, which reached a “B” in the overall rating in the initial round. For these two modules, the rating in the individual categories is therefore not yet published.
The overall rating is highly subjective, as there is no clear criterion for how the individual results have to be weighted. For investors, the detailed results in the individual categories of the test are far more relevant.
Important details
For example, the result in the resistance to potential-induced degradation (PID) is highly relevant to investors. In this category, Jolywood’s module achieves an A rating for system voltages up to 1,000 V. For pv magazine and CEA, awareness of PID resistance is a major issue, and therefore this category is given a 30% weighting in the overall result. PID can be avoided by designing the system with strings grounded at the correct pole. But this is often not the case or not possible. Therefore, it is no surprise that a pv magazine survey published in 2016 revealed that about 16% of the installations are subject to this degradation mode, when the survey results are expanded to apply to all installations in Germany.
When PID occurs, individual modules can lose up to 70% of their performance. The yield loss in plants can range from a few percent to almost total loss. In the past couple of years, many manufacturers promote their modules as “PID-resistant”. One has to be aware, however, that PID is highly dependent on the bill of materials. This may also vary for the same module type. Due to the module selection method employed in this test, whereby a manufacturer can influence the selection process only to a very limited extent, the result concerning PID is an important indication, which makes the marketing statement more credible.
Apart from PID resistance, visual inspection and electroluminescence imaging also play a role in assessing the reliability and lifetimes of modules. In visual inspection, Jolywood achieved a C rating, while in the electroluminescence category an A rating. For comparison, other electroluminescence results that have not yet been published show that some modules have cracks, which are risky. This is not the case with the Jolywood module. However, pv magazine test does not explicitly cover durability tests with climatic chambers, which would make the reliability statement complete. These tests are very extensive and take a comparatively long time.
In contrast, the Pmax temperature coefficient and the low-light efficiency, in which Jolywood scores B and A respectively, do not affect the reliability of a module, but do allow conclusions on the yield that these modules can generate in specified operating conditions. Those who want to accurately predict the yield have to measure the complete module irradiation temperature from different samples of a larger sample size, and then carry out extensive simulations for specific locations. However, some qualitative statements are possible on the basis of the pv magazine test: the low-light behavior is more relevant for areas with frequent cloudy weather and well-marked transitional seasons, where a good fraction of the solar energy is delivered at irradiation levels well below 1,000 watts per square meter. These 1,000 watts per square meter are defined as the standard for the power measurement of the modules. The temperature coefficient, on the other hand, is more relevant for hot climates.
A module with an A rating has on average a 1.6% higher yield compared to a module with a B rating and the same power, measured according to standard test conditions.
The future
pv magazine and the partners in the testing program will continue to develop the tests. As a next step, outdoor performance tests and tests for light-induced degradation are planned. This latter effect is particularly risky for PERC modules. In principle, manufacturers can control this issue. But how well they control it in reality can be shown only through independent testing.
The concept of pv magazine test is to provide the best independent results on module quality under the prevailing market conditions and price pressure. One has to take the results as what they are: another indication, but no prognosis as to how long-lasting and reliable a module will ultimately be. In upcoming issues of , we will continue to report on the results of further modules.
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